Bonding coats for metal



Aug- 5, 1958 P. cUR'rlN t 2;846,;342

BONDING COATS FOR METAL Filed Sept. 30, 1955 Mfr/:Luc 5150557 INVENTOR.

nited States atent Oce :2,846,342 'Patented Aug, 5

2,846,342 y BoNDING coATs Fon METAL Leo P. Curtin, Cranbury, N. J. Application September 30, 1955, Serial No. 537,900 claims. (cl. 14s-6.16)

This invention relates to bonding and oxidation-resisting coats on metal and it comprises wetting the metal surface ywith a solution containing hexavalent chromium, usually in the form of chromic anhydride orV an ammonium salt thereof,` said solution also containing a reducing agent for the hexavalent chromium, the bonding coat being formed on the metal surface by drying the metal atan elevated temperature with reduction of the hexavalent chromium principally to the trivalent state, said trivalent chromium then being present largely in the form of chromic hydroxide in various stages of hydration or dehydration, the bonding coat formed containing substantial amounts of said chromic hydroxide; all asis more fully hereinafter set forth and as claimed.

Whilethe invention is most useful for coating ferrous metal and zinc, it can be used to coat any metal from copper to magnesium in the electromotive scale. This is in-part because the metal being coatedcontributes only in a minor way to the coating. As steel is by far the most important metal, it will be used for purposes of illustration. l

In my co-pending application, U. S. app. S. N. 499,510, there are described Ibonding coats on metal which are of the spinel type, thesebeing produced by the reduction of dichromates of divalent metals present in solution on the surface of the metal, drying and reduction taking place at an elevated temperature. v

The coatings of the present application differ in that they contain substantial amounts of chromic hydroxide, this substance, as denedbelow, being the characteristic and essential element of the coatings of the present invention.

Chromic hydroxide may be written as Cr(OH)3 but, in actual practice, is seldomy found or producedin exactly and eicient, lquantitative reductions being easily attained. Calcium, magnesium and zinc hypophosphites arepa'rticularly desirable and have good water-solubility. v

A large number of other substances will reduce hexavalent chromium under the conditions of the coating reaction, such as, sultes and phyposullites.` Some of these have the disadvantage of leaving a water-soluble residue in the coating.

The use of the ammonumlchromates permits of operating the bath at higher pH levels than when free chromic acid is used. In such cases, `the ammonia is lost from the system by vaporization andthe coatingis `much the same as if chromic anhydride had .been used. Y

The 'following reactions are given to illustrate coating formation:

In (A), chromic anhydride and calcium hypopho'sphite react to produce, principally, chr'omic hydroxide and calcium dihydrogen phosphate. The latter substance is capable of further reactions, forming insoluble substances, as tribasic phosphates. Metal coatings produced by re'- action (A) might be expected to contain upwards of fty percent of chromic hydroxide and a lesser amount of insoluble phosphates.

The number of organic Y:compounds Vwhich can be ,used in the coating reaction isindenitely large, someof the.` better ones being dextrose, glycerol, ethylene glycol, oxalic acid, citric acidfhydrox'yacetic acid and maleic acid. Oxalic acid is the most reactive of the compounds named while ethylene glycol is one of the most con venient and inexpensive.

The coating reaction With ethylene glycol is illustrated as follows: v l

this form. If formed by precipitation at moderate temperatures, it may be in gel form. In practically all cases, the rst product is a chromic hydroxide containing much more water than is shown in Cr(OH)3 and this water is in part heldl tenaciously. For example, it may be heated to 10W-red heat, briefly, without becoming more dehydrated than in the formula just given. For this reason, the chromic'hydroxide of the coatings herein described are believed to range mostly from OCrOH to Cr(OH)3 and various hydrates of the latter compound. In the descriptionand claims the term, chromic hydroxide is intended to include `all such compounds and mixtures thereof. n

As to reducing agents for the hexavalent chromium, there are a great many which are suitable for use in the present invention. One, important group consists of orv ganic compounds 4which are oxidized by hexavalent chromium at elevated temperatures, especially those which oxidize to car-bon dioxide.

Another unusually advantageous group of reducing agents consists of the Oxy-acids ofphosphorus in which the valence ofthe phosphorus is less than live, together with the corresponding ammonium and divalent metal salts. These acids are, hypophosphoric, phosphorous and hypophosphorous, the last-named being the most reactive In (B), chromic anhydride and ethylene glycol interact to give a coating whichJislnearly-all chromic hydroxide.

Part of the ethylene glycol, or other organic reducer, maybe replaced by a hypophosphite'and this willgive a coating containing the corresponding-proportion of phosphate. Or, the bath may contain an addition= of zinc or other divalent dihydrogen phosphate. In such ways, the ratio of phosphate to chromic hydroxide in the coating may be varied over a wide range. Also, small amounts of hypophosphite and phosphite maysometimes befoundinthe coating. f'

In most or all of the coatings, p of other compounds than those indicated, some of these arising from various secondary reactions. v4 usually a slight amount of ferrous iron inthe coating, the iron being derived from the'metal being coated.

The concentration of the chemicals in the coating bath is by no means critical. Good results are obtained with baths containing from about 0.5 to 3.0 percent of chromic anhydride, the other bath components-beingof course based on the chromic anhydride content. As 1will be shown below, there are special types 4of coating procedures with which the chromic acid concentration is decidedly higher than the values' just indicated.

Following are a number of examples .of 4typical coating baths', Aor solutions:

i Total there are smalbamounts There is also.

. 3 The coating of Example lis principally chromic hydroxide.

Example 2 l I Parts by weight Curoniie anhydride v2.0 Zinc-hypophosphite 1.5 Water 'j 96.5

- 1;.' Total 100.0

The `coating of Example 2 isA principally chromic hydixide and'zinc phosphate.

. Example 3 -v l Parts byweight Chrmic anhydride 3.0 Ethylene. zglycol' 0.5 Calcium hypophosphite 0.4 Water 96.1

Total 100.0

The coating of Example 3 is principally chromic hydroxide and calciumvphosphate.

l Example 4 f f. l Parts by weight Ammonium,dichromate 3.0 Magnesiumhypophosphite 1.4 Water 95.6

Total 100.0

The Ycoating of `Example 4 is principally chromic hydroxideand magnesium phosphate.

Exmple 5 f .Y Parts by weight Chrornic lanhydride 2.0 Ethylene glycol 0.5 Zinc-dihydrogen phosphate 1.0 Water 96.5

Total 100.0

The coating is principally chromic hydroxide and zinc phosphate. f

'The coating of YExample '6 is Yprincipally chromic hydroxide, aluminum hydroxide, calcium phosphate and various vproducts of secondary reactions.v

Under commercial conditions, the materials for the coating `solutions may advantageously be supplied in the form ofconcentrated solutions, one of which contains the hexavalent chromium compound and'another solution for the reducing agent. 'The variousmaterial's for use in the baths have adequate water solubilities, some of them, as chroniic anhydride 'and vethylene glycol, being extremely soluble. "The solutions are suiciently stable at room temperature after daddition of 'the lreducing agent to the diluted 'cliromic anhydride solution. The solution of Example' 11,"forfexample, holds up Well 'for a week or more, at the ordinarytemperature. y

- Fa'tty alcohols,'as nido'decyl alcohol,jwhen vmade soluble by mono-esterication with a suitable polyb'asic acid, as maleic or phosphoric, fareiableto act as reducing agents for chromic anhydride. Then, in the form of the correspendinglauryl or. other fatty acid, insoluble salts of diand trivalent metals are-formed, .these entering the coating andenhancing its water-repellentproperties. If -the ester -isof the type of mono-lauryl phosphoric acid, the

4' free phosphoric acid formed by the oxidation also enters the coating as an insoluble phosphate.

It should be pointed out that, in all of the examples above given, except Example 1, there is, in addition to chromic hydroxide, a minor formation of compounds of the spinel type in the coating, these being defined as chromites, ferrites and aluminates of zinc, magnesium, ferrous iron and divalent manganese, also the corresponding alkaline earth compounds, including the numerous complex compounds formed by the interchange of the various radicals. These substances are believed to be produced in small amount by secondary reactions.

The metal sheet, or other metal object, is first cleaned and degreased, as by a hot alkaline bath. It is essential that the surface to be coated be completely freed from water-breaks; otherwise, the coating solution will not wet the affected area and there will be no coating'formation thereon. After rinsing off the alkaline wash, it is-sometimes desirable to subject the surface, especially if of steel, to a very light pickle. This may be accomplished in a variety of ways, for example, by spraying with a 0.75% solution of ferric bisulfate at20 to 50 C., for twenty to forty seconds. This pickling operation is generally quite unnecessary and the coating solution may be applied directly aftcr the water rinse following the alkaline wash. The surface is sprayed or otherwise wetted with cold chromate coating solution, prepared as above described. This can be done, usually, in ten seconds, or less, as it is only necessary completely to wet the surfacey with the coating solution. After draining off the excess chromate coating solution, the kmetal is dried at an elevated temperature, for example, by heating it with a current Iof air or inert gas, preheated to to 200 C., more or less, drying and coating .formation proceeding simultaneously.

This brings about drying and coating formation very quickly, in the case of light sheet, often in less than one minute. The coated metal, after cooling somewhat, may be lacquered at once with no further processing since there is no dust nor non-adherent materialformed.

The metal coatings herein described differ from phosphate and oxalate coatings produced by aqueous solutions in that there is little or no reaction in the coating bath or while the work is being sprayed, consequently, no sludge is produced in the bath. The coating is Yformed almost entirely during the hot drying operation, the coating being derived from the lm of chromate coating solution adhering to the surface.

While the invention is most useful for coating iron and zinc, it can be used to coat any metal from-copper to magnesium in the electromotive series. This is, in part, because the metal being coated contributes only to a very small extent to the coating.

The present invention can be used to coat sheet steel or strip at the time of rolling, with no Vprevious degreasing or pickling. The chromate solution is sprayed on with as little excess as possible, this Vbeing removed by rollers. There is enough residual heat in the sheet metal, if done properly, to evaporate the solvent and produce the coating.

The coatings herein described may be used as a base for fused, vitreous enamels.

Another method for producing the coating is as follows:

The thoroughly cleaned sheet or other metal object is pre-heated to a. fairly high temperature, for example, 250 to 300 C., depending somewhat on the thickness of the metal. The coating solution is then sprayed onto the hot surface in the form of a line mist in an amount' suicient to cover completely the surface while providing little or no runoff. Coating formation is almost instantaneous, the residual heat of the metal providing the temperature for the chemical reactions and the drying of the surface.

This is a most convenient, rapid method of applying bonding Acoats and is particularly.applicableto continuous processing. In addition to the chromic hydroxide coatings of the present invention, it works remarkably well with the process for producing spinel types of coatings, referred to above. It may also be used in coatings which are principally phosphates or oxalates.

Sheet steel, coated in the at with a chromic hydroxide coating as above described, may be lacquered in the at and then stamped out into the desired shape. These stampings may be then subjected to extreme drawing and deforming operations with no impairment of the lacquer and bonding coats, in fact, in extreme cases, it is usually the case that the metal tears before impairment of the coats. lt is of course necessary to select a suitable thermoplastic lacquer, many of the vinyl types being particularly well adapted.

The coatings herein described are very thin, of amorphous or microcrystalline structure, and adhere most tenaciously to the underlying metal. They usually withstand repeated ilexes of 180 degrees.

In color, they are usually some shade from brownish gray to greenish gray. If produced in a hot, inert atmosphere, as ue gas, they may be grayish green, sage green or tea-green.

What I claim is:

l. Process for producing an adherent bonding coat on aluminum, zinc and ferrous metal surfaces which comprises the following sequence of steps: 1) freeing the metal surface from water-breaks, (2) wetting the metal surface with a solution containing chromate selected from the class which consists of chromic acid and ammonium dichromate, the solution also containing a hypophosphite reducing agent for said chromate selected from the class which consists of free hypophosphorous acid and calcium, magnesium and Zinc hypophosphites and (3) destroying substantially completely oxidants and reducing substances present in the lm of said chromate solution adhering to the metal surface by heating to a temperature above 100 C., the residue remaining on the metal surface constituting an adherent, chromic hydroxide-phosphate bonding coat.

2. Process for producing an adherent bonding coat on aluminum and ferrous metal surfaces which comprises the following sequence of steps: (1) freeing the metal surface from water-breaks, (2) wetting the metal surface with a chromate solution, said chromate being selected from the class which consists of chromic acid and ammonium dichromate, the solution also containing an organic reducing agent for said chromate, the solution also containing a phosphate selected from theA class which consists of free phosphoric acid and calcium, magnesium and zinc dihydrogen phosphates, the phosphate being less than the chromate in the solution, and (3) destroying substantially completely oxidants and reducing substances present in the lm of said chromate solution adhering to the metal surface by heating to a temperature above C., the residue remaining on the metal surface constituting an adherent, chromic hydroxide-phosphate bonding coat.

3. The process of claim 2 wherein thereducing agent is selected from the class which consists of polyhydroxy alcohols.

4. The process of claim 2 wherein a part of the reducing agent, about 0.1 to about 0.2 percent based on the weight of the solution, is a phosphoric acid mono-ester of a straight-chain alcohol containing from 10 to 18 carbon atoms.

5. The process of claim 2 wherein the chromate solution is applied to metal preheated to a temperature above 100 C., the heat and temperature necessary to bring about coating formation being supplied by the preheated metal.

6. An article of a metal selected from the group consisting of aluminum, zinc and ferrous metal carrying on tne surface an adherent bonding coat containing up to 49 percentof phosphates of divalent metals selected from the class which consists of calcium, magnesium, zinc and iron, and from 51 to 98 percent of chromic hydroxide, the coating .being substantially free from hexavalent chromium, the coating being capable ofv withstanding deep drawing and repeated flexing through 360.

References Cited in the file of this patent UNITED STATES PATENTS Freud Apr. 25, 

1. PROCESS FOR PRODUCING AN ADHERENT BONDING COAT ON ALUMINUM, ZINC AND FERROUS METAL SURFACES WHICH COMPRISES THE FOLLOWING SEQUENCE OF STEPS: (1) FREEING THE METAL SURFACE FROM WATER-BREAKS (2) WETTING THE METAL SURFACE WITH A SOLUTION CONTAINING CHROMATE SELECTED FROM THE CLASS WHICH CONSIST OF CHROMIC ACID AND AMMONIUM DICHROMATE, THE SOLUTION ALSO CONTAINING A HYPOPHOSPHITE REDUCING AGENT FOR SAID CHROMATE SELECTED FROM THE CLASS WHICH CONSISTS OF FREE HYPOPHOSPHOROUS ACID AND CALCIUM, MAGNESIUM AND ZINC HYPOPHOSPHOROUS AND (3) DESTROYING SUBSTANTIALLY COMPLETELY OXIDANTS AND REDUCING SUBSTANCES PRESENT IN THE FILM OF SAID CHROMATE SOLUTION ADHERING TO THE METAL SURFACE BY HEATING TO A TEMPERATURE ABOVE 100* C., THE RESIDUE REMAINING ON THE METAL SURFACE CONSTITUTING AN ADHERENT, CHROMIC HYDROXIDE-PHOSPHATE BONDING COAT. 